This invention relates generally to signal propagation systems and, more particularly, to such systems in which a signal in the form of radiation, such as electromagnetic radiation, is propagated through a medium or through space, usually between a transmitter and a receiver. It should not be inferred, however, that the invention pertains only to communication systems. As will become apparent from this specification, the invention also has application to other signal propagation systems, such as the use of electromagnetic or particle radiation in the analysis of specimen structure.
In many situations in which an electromagnetic radiation signal is transmitted through a medium (including a vacuum), a relatively high level of energy is used to transmit the radiation signal. Moreover, it is almost uniformly accepted that high energy levels are needed for propagation of the radiation signal. Yet there are many applications in which it would be advantageous to reduce the transmitted energy level. Prior to the present invention, no one has been able suggest how this goal might be accomplished.
A notable example of an application appropriate to the invention is the transmission of data such as text, video, or audio, using electromagnetic radiation. With the prodigious volume of such information now being transmitted over ground-based transmitters and receivers and over satellite links, a substantial reduction in energy usage would be highly desirable. This reduction would be particularly advantageous when applied to a transmitter at a remote site, such as on a satellite, for which electrical power is severely limited. In other situations, it would be equally advantageous to provide a substantially increased signal range for a given power consumption.
Another class of applications relevant to this invention concerns the use of electromagnetic or particle radiation in the analysis of specimen structure. Such applications known in the art are inclusive of virtually the entire electromagnetic spectrum from radio waves through x-rays and of a wide variety of specimens. A specific example is the inspection of manufactured semiconductor structure. It would be desirable to accomplish this task with x-ray inspection beams having -an ultra-low energy content instead of the relatively high energy content that must presently be employed.
In one class of radiography applications, the specimen to be analyzed is subject to damage from energy deposited by an incident radiation beam. A goal that has eluded researchers in this area is to make use of physical properties of a specimen, such as refraction, reflection, or phase shifting, to analyze the specimen without concurrent energy deposition. The ultimate goal in specimen analysis is three-dimensional reconstruction of a specimen image without the use of energy in the incident beam.
It will be appreciated from the foregoing that there are variety of applications using radiation signal propagation for which it would be highly desirable to employ radiation of significantly reduced energy content, without commensurately reducing the detectability of information carried on the signal being propagated. The present invention, as will now be described, accomplishes this goal in an elegant, completely novel and, perhaps, revolutionary manner.
The principal object of this invention is to transmit and receive a radiation signal such as a radio wave in such a way as to significantly reduce the energy content of the signal, but without commensurately reducing the detectability of information carried on the signal. As will become apparent as the description proceeds, the nature of this invention is such that it may provide an important resolution of conflicting fundamental physical theories concerning the nature of electromagnetic radiation. Although this aspect of the invention disclosure may raise interesting, and even controversial, theoretical concerns, it is believed that the detailed structure of the present invention, and verification of its functionality, can be described independently of such theoretical concerns.
The multiplicity of operational uses for the invention yield several distinct but closely related systems. These various operational uses are in fields of application in which the normal energy content of an electromagnetic or particle radiation beam is inherently disadvantageous, or where totally new applications are made possible with significantly reduced beam energy.
The present invention relates to an energy-depleted radiation transmitter and receiver. While it is expedient to summarize the invention in terms appropriate to optical wavelengths, it will be appreciated that the invention is applicable over a wide range of the electromagnetic spectrum, and is equally applicable to particle radiation systems.
For the transmitter, in one of its simplest embodiments, two well-collimated coherent beams, as from a laser with a split beam, are caused to intersect with a small but finite relative angulation along nearly coincident trajectories. At the region of maximum intersection, an interference pattern is formed. This pattern may be a sequence of bright and dark bands. The origin of these bands resides with a lateral migration of energy flux within the intersecting beams toward regions where there is constructive interference and away from regions where there is destructive interference.
In the extreme for two interfering beams, the central region of a bright band may have a nearly doubled energy flux: compared to the average energy flux of the intersecting beams whereas the central region of a dark band potentially may have a nearly zero energy flux. As will soon become clear, the transmitter of the invention propagates radiation from at least one of these dark bands of depleted energy.
Because the invention is functional, it may be inferred that the dark band, although clearly energy depleted, nevertheless still has a wave-like attribute that can carry information, which is the essence of the present invention. The existence of this wave-like attribute may be demonstrated by a simple experiment, which is described in the following paragraphs.
A slit aperture, inserted into the beam path at the location of the interference pattern, initially is aligned with a bright band. The slit should be sufficiently narrow to pass only the central part of a bright band but not so narrow as to produce the Frauenhoffer condition for diffraction. With these restrictions on the slit and appropriate use of lenses in the beam path, the transmitted bright band readily is split into two distinct beams at a selected point sufficiently distant from the slit. This separation is feasible because the transmitted band is a composition of parts of the angularly converging slit-incident beams.
A simple photodetector receiver, situated at the selected distant point, readily measures the energy from the two beam spots on its photon-sensitive surface. The two beam spots appear as a pair of parallel elongated ovals approximately slit-shaped, as expected. The slit is then realigned to a dark band. The slit now appears to transmit nothing and this is apparently supported by an essentially zero reading of the photodetector receiver. Nevertheless, the apparatus is now in a configuration critical to the operation of this invention.
A third beam is split off of the initial coherent beam. Unlike the first two beams, which converged on the slit, the third beam is transmitted directly to the photodetector. The beam spot of the third beam encompasses the entire area where the previous two bright band elongated oval beam spots had been located. The expected energy of the third beam is, of course, measured by the photodetector, but an additional and remarkable phenomenon may be observed at the photodetector. The beam spot of the third beam on the photodetector surface now exhibits two parallel elongated oval interference patterns. Furthermore, these interference patterns are not brighter on average than the rest of the third beam spot, and these patterns disappear when the slit is blocked, leaving only the uniform third beam spot. In accordance with a fundamental aspect of the present invention, the slit in the experiment just described transmits an energy-depleted beam, the wave-like properties of which interfere with the third beam, acting as a reference beam.
These resultant spatial interference patterns are quantified by dividing the photodetector surface into a fine matrix of individually sensitive photodetector elements. These elements must be small enough to sample the separate bright and dark bands on each of the two interference patterns. The differential readings of the neighboring photodetector elements identify the presence and relative contrast of the interference patterns. A detector that can provide such readings of a spatial interference pattern constitutes one embodiment of an interference detector. It may be appreciated that other embodiments of interference detectors known in the art are also applicable to the invention.
In general terms, interference detection combines a signal beam and a reference beam and, with an appropriate detector, measures interference properties of the combined beams. The invention demonstrates that interference properties are still present even when the signal beam is energy-depleted.
The apparatus is adapted to communications by modulating the energy-depleted beams in accordance with a set of data. The modulator may be an electronically variable transmission device of a type known in the art and normally used to control the intensity or the phase of a photon beam. The invention is also applicable to communications in signal processing systems, such as high speed computers, where energy density is a limiting factor, since the signal beam in the invention transmits information with very little energy.
The invention is also adaptable to communications where only the modulated energy-depleted beams are transmitted over the intervening space to a distant receiver detector. In this application, the required reference beam is generated with the same wavelength as the signal beam by an independent coherent source in the locality of the receiver. Transitory mutual coherence of the modulated energy-depleted beams and the independent source requires sufficiently rapid response time of receiver detector elements.
Transmission media appropriate to the transmission of energy-depleted beams are the same as those used for the corresponding radiation not depleted in energy. For example, energy-depleted radio wave radiation can be transmitted through vacuum or atmosphere. Similarly, energy-depleted light can be transmitted on fiberoptic cables.
The apparatus is adaptable to analysis of a specimen by directing the energy-depleted beams at the specimen and assessing the resultant interference contrast or phase shift. This application permits the analysis of a specimen without unwanted energy deposition in the specimen.
Briefly, and in general terms, the energy-depleted radiation generator of the invention comprises at least one conventional source of radiation providing at least one coherent beam of energy-bearing radiation; and at least one beam interaction element, including at least one interaction region into which is input the beam of energy-bearing radiation, and from which is derived at least one energy-depleted radiation beam having wave properties identical to those of the conventional source of radiation. More specifically, the conventional source of radiation in one embodiment of the generator provides first and second coherent radiation beams, and the beam interaction element includes means for directing the first and second coherent radiation beams along paths that intersect in the interaction region and produce an interference pattern with zones of constructive interference and zones of destructive interference. The beam interaction element also includes means for selectively transmitting energy-depleted radiation from at least one zone of destructive interference in the interaction region. The means for selectively transmitting energy-depleted radiation includes a transmissive aperture positioned in the interaction region and aligned with a zone of destructive interference. The aperture transmits intersecting out-of-phase energy-depleted radiation that diverges beyond the aperture into individual spatially separated beams of in-phase energy-depleted radiation.
In another embodiment of the generator, the conventional source of radiation provides first and second coherent radiation beams and each of the beam interaction elements includes a photorefractive device and means for directing the first and second coherent radiation beams into the photorefractive device in such a manner that energy is coupled from one beam to the other in the photorefractive device, resulting in output of an energy-depleted beam and an energy-enhanced beam. The generator may include a plurality of similar photorefractive devices coupled together in a series chain such that the energy-depleted beam and the energy-enhanced beam from one photorefractive device are input to a next photorefractive device, and wherein the series chain of photorefractive devices provides successively greater levels of energy depletion in the energy-depleted beam.
In yet another embodiment of the generator, the conventional source of radiation provides a single radiation beam, and the beam interaction element includes at least one directional coupler having first and second waveguides. The single radiation beam is input to the first waveguide and the directional coupler is configured to transfer energy from the first waveguide to the second waveguide, leaving an energy-depleted beam for output from the first waveguide. Optionally, the generator may include a plurality of similar directional couplers connected together in multiple stages such that an energy-bearing beam from the second waveguide of one directional coupler is connected as the first waveguide of a next directional coupler. The multiple stages of directional couplers provide multiple outputs of energy-depleted beams from the first waveguide in each stage.
Any of these generators of energy-depleted radiation may also include a radiation modulator, for selectively modifying a property of the energy-depleted radiation. The modulator may include means for encoding data onto the energy-depleted radiation, or may take the form of a specimen on which the energy-depleted radiation impinges.
Another important aspect of the invention resides in a device for restoring energy to energy-depleted radiation. Most generally, this energy restoring device includes an external source of energy coupled to an energy-depleted radiation beam in an interaction element.
In several embodiments of the energy restoring device, the external source of energy is a reference beam coherent with an energy-depleted radiation beam and the device includes a beam interaction element having an interaction region, means for directing the energy-depleted radiation beam and the reference beam into the interaction region, and means for directing energy-restored radiation from the interaction region.
In one embodiment of the energy restoring device, the interaction region of the beam interaction element includes an interference zone, and interference of the energy-depleted beam and the reference beam produces interference bands of energy-restored radiation. The means for directing energy-restored radiation from the interference zone produces an energy-restored output beam having identical wave attributes to the input energy-depleted beam.
In another embodiment of the energy restoring device, the beam interaction element includes a two-beam coupler in which energy is coupled from the reference beam to the energy depleted beam, producing an energy-restored output beam having identical wave attributes to the input energy-depleted beam. This embodiment of the device may further include at least one additional two-beam coupler, for restoring additional energy to the energy-restored output beam from the first two-beam coupler.
In other embodiments of the energy restoring device, the beam interaction element includes an optical amplifier. In one embodiment, the optical amplifier consists of a device having two input ports through which an energy-providing conventional shorter wavelength beam and the energy-depleted beam are introduced, and having an output port for an energy-restored beam derived by coupling energy from the shorter wavelength beam to the energy-depleted beam. In another embodiment, the optical amplifier consists of a device in which input electrical energy is coupled to the input energy-depleted beam to provide for an output energy-restored beam. Optionally, for either embodiment, the device further includes at least one additional optical amplifier, for restoring additional energy to the energy-restored output beam from the first optical amplifier.
It will be appreciated from the foregoing that the receiver of energy-depleted radiation in this invention uses interference detection of the energy-depleted radiation beam or, alternatively, energy-restoration of the energy-depleted radiation beam with subsequent conventional detection of the energy restored beam.
The invention, which may also be expressed in terms of a method, has other embodiments combining the energy-depleted generators, detectors, energy-restorers, modulators and demodulators to achieve more specific goals. As well as communications, and specimen analysis, the invention may also be applied to the retrieval of recorded information from a photorefractive medium, and to a process for making holographic recordings. Both of these applications benefit from the use of energy-depleted radiation.
It will also be appreciated from the foregoing that the present invention represents a significant, if not revolutionary, advance in the propagation of signals as radiation. A number of embodiments of the invention have been mentioned in this summary. Further variations of the apparatus and method of the invention will become apparent from the following more detailed description, including embodiments of the invention applied to communication systems, specimen analysis systems, and holographic and photorefractive recording systems. It will be appreciated from the description that follows that the invention is applicable to particle radiation systems, such as systems using electron radiation, as well as to electromagnetic radiation systems operating at various wavelengths.